Contents Introduction  295 Evolution of the Plant Vascular System  295 Phloem Development & Differentiation  300 Molecular Mechanisms Underlying Xylem Cell Differentiation  307 Spatial & Temporal Regulation of Vascular Patterning  311 Secondary Vascular Development  318 Physical and Physiological Constraints on Phloem Transport Function  321 Physical & Physiological Constraints on Xylem Function  328 Long‐distance Signaling Through the Phloem  339 Root‐to‐shoot Signaling  347 Vascular Transport of Microelement Minerals  351 Systemic Signaling: Pathogen Resistance  356 Future Perspectives  361 Acknowledgements  362 References  362 Abstract [ William J. Lucas (Corresponding author)] The emergence of the tracheophyte‐based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long‐distance communication system are next assessed in terms of the coordination of developmental, physiological and defense‐related processes, at the whole‐plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state‐of‐the‐art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

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Different root parts with or without increased iron-reducing activities have been studied in iron-deficient and iron-sufficient control sugar beet (Beta vulgaris L. Monohil hybrid). The distal root parts of iron-deficient plants, 0 to 5 mm from the root apex, were capable to reduce Fe(III)-chelates and contained concentrations of flavins near 700 m, two characteristics absent in the 5 to 10 mm sections of iron-deficient plants and the whole root of iron-sufficient plants. Flavin-containing root tips had large pools of carboxylic acids and high activities of enzymes involved in organic acid metabolism. In iron-deficient yellow root tips there was a large increase in carbon fixation associated to an increase in phosphoenolpyruvate carboxylase activity. Part of this carbon was used, through an increase in mitochondrial activity, to increase the capacity to produce reducing power, whereas another part was exported via xylem. Root respiration was increased by iron deficiency. In sugar beet iron-deficient roots flavins would provide a suitable link between the increased capacity to produce reduced nucleotides and the plasma membrane associated ferric chelate reductase enzyme(s). Iron-deficient roots had a large oxygen consumption rate in the presence of cyanide and hydroxisalycilic acid, suggesting that the ferric chelate reductase enzyme is able to reduce oxygen in the absence of Fe(III)-chelates.